1. Field of the Invention
The present invention relates to surface pretreatments and, more specifically, to a system for pretreating large-area plastics and similar materials to improve adhesion properties of coatings.
2. Description of the Related Art
There are numerous methods used to activate or pretreat plastics (e.g. polymers) to improve the adhesion of coatings such as metals. One of the simplest and most effective methods is to roughen the surface by sandblasting, grinding, or chemically etching to increase the contact area of the surface. Chemical etching is particularly attractive because it provides selectivity over various polymer phases; however, the handling of the hazardous chemicals typically used in this approach is undesirable. Other techniques to enhance adhesion of polymers are flame treatment, corona discharge and low pressure plasma activation. All of these pretreatment techniques are plasma-based and form a broad variety of oxygen containing functional groups on the polymer surface. During such treatments, the near-surface layer can also be cross-linked by the ultraviolet (UV) radiation emitted by the plasma, which degrades the polymer surface and in turn decreases the surface adhesion capabilities.
The surface chemistry of polymers is very important in their adhesion properties; hence, plasma treatments, which offer a broader range of reactions, have been used in industry and research. Generally, adhesion can be greatly altered by certain functional groups on particular material surfaces. For instance, the adhesion of aluminum to polyester (PET) is due to the inherent ether (C═O) groups which in turn form metal oxide (C—O—Al) bonds. The number of these metal oxide bonds can be increased by surface oxidation via plasma treatment, and thus increase the strength of the Al/PET interface. However, the same PET substrate becomes extremely hydrophobic (water-repellant) due to the increased polarity of the additional surface moieties (functional or radical groups) and the micro-roughening (etching) produced by the plasma. The balance between these properties comes from the ability to control the pretreatment process in order to achieve the desired outcome. As another example, to ‘metallize’ a non-oxygen containing material, a plasma pretreatment containing oxygen may be used to produce radical species, which activate and also react with (e.g. oxidize) the surface. The metal adhesion is similarly increased through metal oxide bonds. If the surface is to be prepared for a different treatment, the plasma parameters can be adjusted during the pretreatment to produce increased surface roughening (e.g. sputtering by plasma ions), different surface moieties (e.g. plasma production of different radical and ion species) or cross-linking (e.g. various UV light exposure).
Various plasma sources have been used to demonstrate changes in plastic and polymer surface structure and the effects of plasma processing on the adhesion coatings. Most of these systems have been very simple arrangements that consisted of rf and dc glow discharges. The shortcoming of such systems has been (1) the inability to control plasma species and thus target specific surface reactions, (2) excessive substrate heating by the plasma source, which can be detrimental to many materials, and (3) over exposure of UV radiation from the plasma source. Furthermore, scaling up to large processing areas (e.g. >1 m2) while retaining process uniformity and quality is not necessarily achievable.